Notable characteristics

Chromium is a steel-gray, lustrous, hard metal that takes
a high polish, is fusible with difficulty, and is resistant
to corrosion and tarnishing.

The most common oxidation states of chromium are +2,
+3, and +6, with +3 being the most stable. +4 and +5 are
relatively rare. Chromium compounds of oxidation state
6 are powerful oxidants.

Applications

Uses of chromium:

In metallurgy, to impart corrosion resistance and
a shiny finish:

as an alloy constituent, e.g. in stainless steel,

in chrome plating,

in anodized aluminium (literally turning the surface
of an aluminium part into ruby).

As a catalyst.

As a metal polish Cromium 3 Oxide is known as Green
rouge.

Chromite is used to make molds for the firing of bricks.

Chromium salts color glass an emerald green.

Chromium salts are used in the tanning of leather.

Chromium is what makes a ruby red, and therefore is
used in producing synthetic rubies.

The chromates and oxides are used in dyes and paints.

Potassium dichromate is a chemical reagent, used in
cleaning laboratory glassware and as a titrating agent.
It is also used as a mordant (i.e. a fixing agent) for
dyes in fabric.

Chromium Dioxide (CrO2) is used to manufacture
magnetic tape, where its higher coercivity than iron
oxide tapes gives better performance.

History

In 1761, Johann Gottlob Lehmann found an orange-red mineral
in the Ural Mountains which he named Siberian red lead.
Though misidentified as a lead compound with selenium
and iron components, the material was in fact a lead chromate
(PbCrO4).

In 1770, Peter Simon Pallas visited the same site as
Lehmann and found a red "lead" mineral that had very useful
properties as a pigment in paints. The use of Siberian
red lead as a paint pigment developed rapidly. A bright
yellow made from crocoite became a very fashionable color.

In 1797, Nicolas-Louis Vauquelin received samples of
crocoite ore. He was able to produce chromium oxide (CrO3)
by mixing crocoite with hydrochloric acid. In 1798, Vauquelin
discovered that he could isolate metallic chromium by
heating the oxide in a charcoal oven. He was also able
to detect traces of chromium in precious gems, such as
ruby, or emerald.

During the 1800s chromium was primarily used as a component
of paints but now the primary use (85%) is for metal alloys,
with the remainder used in the chemical industry and refractory
and foundry industries

Chromium was named based on the Greek word "chroma" meaning
color, because of the many colorful compounds made from
it.

Biological role

Trivalent chromium is an essential trace metal and is
required for the proper metabolism of sugar in humans.
Chromium deficiencies can affect the potency of insulin
in regulating sugar balance. Unlike other essential trace
metals, chromium has not been found in a metalloprotein
with biological activity. Therefore, the functional basis
for the chromium requirement in the diet remains unexplained.

Occurrence

Chromium is mined as chromite (FeCr2O4)
ore. Chromium is obtained commercially by heating the
ore in the presence of aluminium or silicon. Roughly half
the chromite ore in the world is produced in South Africa.
Kazakhstan, India and Turkey are also substantial producers.
Untapped chromite deposits are plentiful, but geographically
concentrated in Kazakhstan and southern Africa. Approximately
15 million tons of marketable chromite ore were produced
in 2000, and converted into approximately 4 million tons
of ferro-chrome with an approximate market value of 2.5
billion US dollars.

Though native chromium deposits are rare, some native
chromium metal has been discovered. The Udachnaya Mine
in Russia produces samples of the native metal. This mine
is a kimberlite pipe rich in diamonds, and the reducing
environment so provided helped produce both elemental
chromium and diamond.

Compounds

Potassium dichromate is a powerful oxidizing agent and
is the preferred compound for cleaning laboratory glassware
of any possible organics. Chrome green is the green oxide
of chromium, Cr2O3, used in enamel
painting, and glass staining. Chrome yellow is a brilliant
yellow pigment, PbCrO4, used by painters.

Chromic acid has the hypothetical structure H2CrO4.
Neither chromic nor dichromic acid is found in nature,
but their anions are found in a variety of compounds.
Chromium trioxide, CrO3, the acid anhydride
of chromic acid, is sold industrially as "chromic acid".

Isotopes

Naturally occurring chromium is composed of 3 stable
isotopes; 52-Cr, 53-Cr, and 54-Cr with 52-Cr being the
most abundant (83.789% natural abundance). 19 radioisotopes
have been characterized with the most stable being 50-Cr
with a half-life of (more than) 1.8E17 years, and 51-Cr
with a half-life of 27.7025 days. All of the remaining
radioactive isotopes have half-lifes that are less than
24 hours and the majority of these have half lifes that
are less than 1 minute. This element also has 2 meta states.

Chromium-53 is the radiogenic decay product of 53Mn.
Chromium isotopic contents are typically combined with
manganese isotopic contents and have found application
in isotope geology. Mn-Cr isotope ratios reinforce the
evidence from 26Al and 107Pd for
the early history of the solar system. Variations in 53Cr/52Cr
and Mn/Cr ratios from several meteorites indicate an initial
53Mn/55Mn ratio that suggests Mn-Cr
isotope systematics must result from in-situ decay of
53Mn in differentiated planetary bodies. Hence
53Cr provides additional evidence for nucleosynthetic
processes immediately before coalescence of the solar
system.

The isotopes of chromium range in atomic weight from
43 amu (43-Cr) to 67 amu (67-Cr). The primary decay mode
before the most abundant stable isotope, 52-Cr, is electron
capture and the primary mode after is beta decay.

Precautions

Chromium metal and chromium(III) compounds are not usually
considered health hazards, but chromium (VI) compounds
can be toxic if orally ingested. The lethal dose of poisonous
chromium (VI) compounds is about one half teaspoon of
material. Most chromium (VI) compounds are irritating
to eyes, skin and mucous membranes. Chronic exposure to
chromium (VI) compounds can cause permanent eye injury,
unless properly treated. Chromium(VI) is an established
human carcinogen.

In 1958 the World Health Organization recommended a maximum
allowable concentration of 0.05 mg/litre in drinking water
for chromium (VI), based on health concerns. This recommendation
has been reviewed a number of times and this value has
not been revised in the meantime.

As chromium compounds were/are used in dyes and paints
and the tanning of leather, these compounds are often
found in soil and groundwater at (abandoned) industrial
sites, now needing environmental cleanup and remediation.
See also brownfield land.